High-resolution (p,t) study of low-spin states in Pu-240: Octupole excitations, alpha clustering, and other structure features

Background: Many nuclear-structure features have been observed in actinides in recent decades. In particular, the octupole degree of freedom has been discussed lately after the successful measurement of the B(E3; 0(1)(+) -> 3(1)(-)) reduced transition strength in Ra-224. Recent results stemming from gamma-spectroscopy experiments and highresolution (p, t) experiments suggested that strong octupole correlations might be observed for some positiveparity states of actinide nuclei. Purpose: This work completes a series of (p, t) experiments on actinide nuclei by adding the data on Pu-240. The (p, t) experiments allow us to study low-spin states up to J(pi) = 6(+). Besides two-nucleon transfer cross sections, spin and parity can be assigned to excited states by measuring angular distributions, and several rotational bands are recognized based on these assignments. Methods: A high-resolution (p, t) experiment at E-p = 24 MeV was performed to populate low-spin states in the actinide nucleus Pu-240. The Q3D magnetic spectrograph of the Maier-Leibnitz Laboratory (MLL) in Munich (Germany) was used to identify the ejected tritons via dE/E particle identification with its focal-plane detection system. Angular distributions were measured at nine different Q3D angles to assign spin and parity to the excited states based on a comparison with coupled-channel distorted-wave Born approximation calculations. Results: In total, 209 states have been excited in Pu-240 up to an excitation energy of 3 MeV. Many previously known states have also been observed and their spin-parity assignments were confirmed. However, many of the populated states have been seen for the first time, e.g., 15 new and firmly assigned J(pi) = 0(+) states. In addition, all low-spin one-octupole phonon excitations, i.e., K-pi = 0(-), 1(-), 2(-), 3(-), could be observed and a new candidate for the K = 3 projection is proposed. Furthermore, the double-octupole or a-cluster structure of the 0+2 state in Pu-240 has been studied in more detail. It is shown that the 0+2 state in Th-230 has a distinctly different structure. In addition, strongly excited 1-states have been observed at 1.5 and 1.8 MeV in Pu-240. The present study suggests that similar states might be observed in Th-230. Conclusions: At least two different and distinct structures for J(pi) = 0(+) states are present in the actinides. These are pairing states and states with enhanced octupole correlations. We have shown that it is crucial to consider negative-parity single-particle states being admixed to some K-pi = 0(2)(+) rotational bands to understand the alpha-decay hindrance factors and enhanced E1-decay rates. Based on our analysis, we have identified the double-octupole or alpha-cluster K-pi = 0(+) candidates from Ra-224 to Pu-240